Step-by-Step selecting and moving vertices to absolute positions
In the 3D view, press N once or twice to open the
right-hand sidebar.
Find the '3D Cursor' Section and set the Z value to your desired
value, 0.2 in this case.
At the bottom of the 3D view, set the Pivot Center to '3D Cursor'.
Drag select your vertices with B.
Hit S (for scale), then Z (to limit scaling
along the z-axis), then 0 (zero).